Ambush Blade

You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects and an invisible blade appears behind the target. If the target is hit and damaged by a melee attack from another creature before the start of your next turn, it takes 1d10 force damage from the ambush blade.

This spell’s damage increases when you reach certain levels. At 5th level, the damage the target takes from the secondary effect increases by 2d10. The secondary damage increases by 2d10 again at 11th level (5d10) and at 17th level (7d10).